Don't you run into limitations of raspberry's real time capabilities with a task like this?
A REAL, microcontroller based, appliance, which is connected through a serial port to any PC, would be more suitable.
The most important thing though is getting rid of the PC where it isn't necessary and making the whole thing an appliance. Otherwise the PC will be the weakest link in the system. It can be replaced but it's expensive and cumbersome (updates, security, etc) if all you want is the appliance for balancing axles.
I doubt there are any difficult hardware requirements for a task like this, if it's like balancing wheels it's no harder than (slow) analog sampling. I have done it with a cheap analog io card on a 386 with DOS for regular auto shop balancing.